This invention relates to optical communication networks and particularly, but not exclusively, optical fibre communications networks for distributing optical signals from transmitting stations to one or more receiving stations.
In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
One approach to transferring data from a transmitting station to many receiving stations is to transmit the data for the respective receiving stations in a dedicated time slot on a common optical carrier of a particular wavelength. The optical signal is then passively distributed via an optical waveguide network to each of the receiving stations which extracts data from the appropriate time slot. An alternative approach is to transmit the data for each receiving station on a distinct wavelength. The wavelength multiplex so formed is again passively split to each receiving station which demultiplexes the appropriate wavelength to receive the data intended for it.
In each of these approaches, and others which rely on passive splitting of an optical signal to two or more receiving stations, it is sometimes necessary to amplify the optical signal to compensate for losses such as absorption and insertion losses and the signal power diverted to a receiver to ensure that there is sufficient optical power for receivers further downstream. This has been achieved by amplifying the optical signal by means of optical amplifiers at one or more positions along the network for example using a power amplifier at the transmitter, repeater amplifiers, or preamplifiers at the receivers.
Known amplifiers employing optically pumped optical amplifiers employ dedicated optical pump sources colocated with the optical amplifier. See for example EP 248,517 A2 in which one or more portions of Raman-active optical fibre adapted for amplification of electro-magnetic radiation by means of stimulated Raman scattering, each portion having associated with it a local optical pump. For a large network this can result in a large number of widely distributed electro-optical units or laser sources which are relatively expensive compared to the rest of the optical network. Further, the failure rate of all or part of the network will increase with the number of such repeaters.